Process of forming non-woven porous fibrous synthetic leather sheet



Uniad w a m o w PROCESS OF FORMING NON-WOVEN PQ QUS FIBROUS SYNTHETICLEATHER j-Jolm Augustus Piccard, Swarthmore, Pa., and Bnynton G aham.larmp tl fi .a si no to du Pont 3 (le Nernours and Company, Wilmington,Del a corpor'ation of Delaware No Drawing. Application July 29, 1952,Serial No. 301,603

5 Claims. (Cl. 18-48.)

I This invention relates to synthetic leather and, more particularly, tothe process of treating a non-wovenpolyvise a simple and rapid techniqueof producing. synthetic .leather cornpositions. In mostof the early art,pyroxylin was used to coat or impregnate various types of fibrous basematerials to prepare leather substitutes. As time wenton,pyroxyIin/oil/pigment compositions were. widely used as coating. orimpregnating compositions for various woven or non-woven fibrous basematerials. In the early stages of the synthetic leather industry,themain objective was to simulate the general appearance 10f leathetn, 1

InItodays markets, coated fabrics, particularly the vinyl-coatedfabrics, are outstanding as leather substitutes in such applications ashandbags, bookbindings, brief cases, card table covers, luggage, etc. Insuch applications, the coated. fabrics are satisfactorywbecause:the

'generaLappearance of leather is. simulated; and the coated 'jfabricspossesssome of the desirable propertiesofleather.

However, as compared to leather, the coated-fahricsrlack good tearstrength, softness and the ability to breathe" or transpire water vaporand air; and although the coated fabrics are used in such applicationsas chair coverings, much is left to be desired, especially with respectto water vapor and air permeability. Up to the present time, 1

synthetic leather compositions have made little or no inroads into theboots, shoes, and gloves marketspm rinly because of their inability tobreathe in addition to l'ack of good tear strength and softness. As usedhereinafter, the term breathe means transpire water vapor andair.

In general, the use of a synthetic leather composition in boots, shoes,gloves, etc., is mainly dependent upon its ability to breathe, usuallyexpressed in terms of water temperature of the binder material.

vapor permeability or leather permeability. Phyical tests on the watervapor permeability of leather indicate 7 that leather transpires watervapor about /3 as readily as free air. In general terms, shoe upperleather samples having a thickness of 0.016"-0.104" have a leatherpermeability within the range of LOGO-18,000 gms./ 100 sq.umeters/hr.when tested according toKanagy &Vickers Journal of American LeatherChemical Association 45,

211 -2421 (.April 1950), in an atmosphereof 23: C. and

890% relative humidity. Hereinaft'er,-the ability of syn- =theticleather compositions 1 to transpire waterwaporwill =be expressed interms of leather permeability in gins/100 sq; meters/hr. 1 Based uponcomforttests, the minimum tolerableleather permeability for shoeupperleather is about 2,000 gms./ 1 00 sq. meters/hr. Preferably, for sheeupper leather, the permeability value should be 4,000-20000 when testedat 23 C., and not greater than relative humidity.

fin- 'ohject of the, present invention is to provideasym ,1:

thetic leather having outstanding breathing qualities. A further objectof the present invention is to provide a process of preparing asynthetic leather having the requisite properties for fabricating boots,shoes, gloves, garments, chair coverings, and other articles wherein acomposition capable of breathing is required. A still further object isto provide a process of preparing a synthetic leather having a tenacity,flex life, elongation, tear strength, modulus and leather permeabilityequal or superior to the various types of genuine leather. Other objectswill be apparent from the description given hereinafter. i

The above objects are accomplished according to the present invention byforming a compact, essentially impermeable, and continuous compositesheet by hot pressing, a composition comprising a structural fiber component, an extractable pore-forming fiber component and a bindermaterial, the proportion of pore-forming fiber, based upon the totalvolume of the sheet, being from 4 0570%, the weight ratio of structural.fiber to, binder material being from 1:2 to 2:1, and thereafter forminginterconnecting poreshaving essentially the shape of the .porerform ing.fibers in thesheet by extracting the poreforming fiberQ t The preferredstructural fiber is an, orientedsynthetic linear polyamide, for example,polyhexam'ethylene adiparnide, polyhexamethylene sebacamicle,polycaproamide, or an interpolyamide of the type disclosed in U,.S.'P.2,285,009. IThePore-forming fiber is one. which ;is readily extractablewith a solvent, e. g., waiter, acetone, etc.,. that is substantiallyinert, i. e., hasfno solvent action, toward ,the structural fiber andthe binder material. The pre ferred p ore-forming fibers are celluloseacetate. and polyvinyl alcohol. The binder material may beselectedffrorn the large class of soft, elastic, at least initiallythermoplastic, synthetic polymers, the preferred polymers beingN-methoxymethyl polyhexamethylene adipamide, polyethylenetand its.derivatives, copolyesters made from ethylene glycol and 60 mol per centof terephthalic acid and 40 mol per cent of sebacic acid, syntheticrubbers such as neoprene (polymeric 2-chloro-1,3-butadiene) andcompositions containing vinyl chloride polymers or copolymers. i l i iThe pore-forming fiber and the structural fiber componentsmust havesoftening temperatures above the flow Furthermore, the structural-fibercomponent and the binder material must be insolublein the solventused'to extract the pore-form- "ing'fiber component.

The following-examples will serve to illustrate the principles andpractice of the present invention and demonstrate the fcriticalness of.the specified relative proportions of ingredients employed. Parts are byweights unless otherwise indicated.

Example 1 A mixture of 1.688 parts of 0.01 average length, 3denier/filament, polyhexamethylene adipamide staple and 4,125 parts, of0.01" average length, idenier/filament, celluloseacetate staple wasdispersed in, 600 parts of water by'stirring. To the dispersionwas added12.05 parts of a 14% latex of N-methoxyrnethy1 polyhexamethyl eneadipamide having a DV (DV=dllution value, as defined in U. S. P.2,430,923) of 55, together with 0.01 part of ootyl phenyl polyglycolether (to maintain the fiber dispersion) and 0.02 part of acetic acid(to fleeculate the polyamide dispersion). After stirring briefly, 'a matwas formed by filtration through coarse filter paper, the area of thefilter being such as to provide a retained weight of fibers plus'biuderof 3.0 oz./sq.,.ft. The. mat was dried and pressed for five: minutes at14 0 C. under a pressure of 1930 lbs/sq. in, andthe cellulose 3 acetatecomponent was then removed by extracting the mat for 18 hours with warmrunning acetone. The extra-cted product weighed 1.34 oz./sq. ft. and wasleatherlike in appearance and feel. It had a tongue-tear strength(average resistance to propagation in the lnstron tester) of 1.6 pounds.(Tongue-tear strength is measured by cutting a 1" slit in the sheet tobe tested and thereafter measuring the average force in pounds requiredto propagate the tear. The test is carried out in a manner similar toASTM test method D39-39.) The product exhibited a leather permeabilityof 2369 gms./100 sq. meters/hr. when tested at 23 C. and 90% relativehumidity. (In similar tests of water vapor permeability, various samplesof representative shoe upper leathers exhibited the following leatherpermeabilities, all expressed in gms./100 sq. meters/hm Suede calf13,932 Glazed kid 3,718 Scotch grain cow 6,161 Cretan butt 7,863 Englishcalf 9,430

A considerable series of actual wearing comfort tests has indicated thatmembranes exhibiting a permeability in this test of 2,000 to 10,000 willprovide wearing comfort equivalent to that obtained with glazed leathersand heavy shoe upper leathers, while permeabilities of 10,000 to 22,000were found to provide wearing comfort equivalent to that obtained withthe lightest shoe upper leathers.) The leather permeability test devisedby Kanagy & Vickers, as described in the aforesaid article, was found tocorrelate with wearing comfort tests. In this test, a 3-inch diametercrystallizing dish was filled with 12-mesh calcium chloride, coveredwith a membrane of the substance under test, suspended inverted in anatmosphere of high humidity, and weighed at intervals. All values ofleather permeability were measured at 23 C. and 90% relative humidity.

Example 2 A non-woven, fibrous, composite sheet was prepared by pressingthe following plies of carded fibers into a composite sheet:

soap and shoe polish, the material had a leather permeability of 7151and a resistance to penetration by liquid Water of 36 flexes, whentested according to the procedure descnibed in Federal LeatherSpecification KK-L-3l1, Method #271.1 (3/28/45). Representative shoeupper leathers have given the following valuesin this test:

Example 3 A non-woven fibrous sheet was prepared in a manner similar tothat of Example 2. The top ply was composed of 40 parts of thecopolyester staple binder of Example- 2 and 30 parts of celluloseacetate fiber pore-former, 3 denier/ filament, 1.5 in length. The middleply was composed of parts of 3 denier/filament, 2.5 polyhexamethyleneadipamide structural fiber, 60 parts of the copolyester binder fiber,and 180 parts of cellulose acetate pore-former. The base layer wascomposed of 60 parts of the polyamide structural fiber, 60 parts of the00- polyester binder fiber, and 240 parts of the cellulose acetatepore-former. The composite structure contained an overall ratio ofstructural fiber/binder/pore-former of l6.5/2l.9/61.6. The product was0.034" thick and weighed 1.3 oz./sq. ft It had a leather permeability of14,284, a tensile strength of 1,739 lbs/sq. in., an elongation of 105%,a modulus of 3,101 lbs/sq. in., a tonguetear strength of 10.6 pounds,and a Schiltknecht flex life of 826,0004,645,000 flexes. Afterapplication of saddle soap and shoe polish, the product had a leatherpermeability of 7,555 and a Schiltknecht flex life of 1,677,000-28,000,000.

Example 4 This example illustrates the pertinent physical properties ofthe initial non-woven, fibrous, impermeable structure of the presentinvention prior to extraction of the pore-forming fiber.

Each ply was prepared by carding the fibers into a bat,

and the plies were pressed together in cross-grain fashion is describedin Bulletin #105 of Alfred Suter, 200 Fifth Avenue, New York, New York),and a cold crack temperature of below -70 C. After application of saddleA product was prepared as described in Example l except that the step ofextracting the pore-forming cellulose acetate staple was omitted. Theproduct weighed 3.0 oz./sq. ft., and it was very stiff and notleather-like in appearance or handle. It had a leather permeability ofonly 84 gms./ sq. meters/hr., a tongue-tear strength of only 1.0lb./oz./sq. ft., and a Schiltknecht flex lifeof only 11,875 flexes. As amatter of comparison, the product described in Example 1 had atongue-tear strength of 1.20 lbs./oz./sq. ft.; and a product similar tothat described in Example 1 had a Schiltknecht flex life of25,000-66,000. Thus, the step of extracting the poreforming fibrouscomponent improved not only permeability to water vapor, but also tearstrength and flex life.

Example Amat similar-to thatd scribed in E a pletl was prepared exceptthat the ,ratio, of components was varied to provide a polyhexamethylenea-dipamide fiber/N-methoxymethyl polyhexamethylene adipa-mide binderfiber/cellulose acetate pore-former fiber ratio of 15/30/55 (structuralfiber/binder ratio of -1/2,). The product was {permeable and had atongue-tear strength of 1.1 pounds.

Example 6 when the fiber/binder ratio was less than 1/2, or greate than2/1. As will be illustrated hereinafter, the tear strength ofthesynthetic leather compositions of the pres,- ent invention will varywith the length of the structural fiber. In Examples 1, 5. and 6, thelength of thestruetural fiber was 0.01", a minimum value insofar asfabricating structures of substantial tear strengthh is concerned. Ingeneral, experience has shown that little additional strength isobtained by employing a structural fiber longer than about 1.5".l-Iowevenit may be more convenient,

from the standpoint of the type of textile machinery to I be employed,to use structural fibers from 0.5" to 8" in length. Hence, employing afiber/binder ratio of f rom l/2 to 2/1 produces a composition having anoptimum tear strength. This is a surprising discovery on the basis ofwhat was known heretofore about fiber-reinforced compositions.

The following examples (7-15 inclusive) illustrate the effect of varyingthe contentof pore-forming fiber in the initial composition upontheleather permeability and tear strength of the extracted product.

Example 7 A matwas prepared as described inExamplel except that theratio of structural fiber/binder/pore-formerwas intheratio of37.5/37.5/25. The product had a leather permeability of 496 and atongue-tear strength of 33.6 pounds.

Example8 A mat similar to that of Example Twas prepared except that thestructural fiber/binder/pore-for-mer was in the ratio of 30/ 30/40. Theresulting product had a leather permeability of 588.

Example'9 A mat similar to that of Example 7 was prepared Except, thatthe ratio of structural 'fiber/binder/porefOrmer was, in the ratio of27.5/27.5/45. The extracted product had a leather permeability of 3,680.

Example 10 A mat similar to that of Example 7 was prepared except thatthe ratiotof structural fiber/binder/poreformer was in the ratio of25/25/50. The product had a leather permeability-of 8,,85I3randatougue1tear-.-stre gth ofi'2l7poundsy g E ample 1-1 A mat similar toExample 7 was prepared except that the ratio, of structuralfiber/binder/pore-former 15115170. The extracted product had a leatherpermeability ot 17,895 and a tongue-tear strength of 1.4 pounds.

Example 13 A mat similar tothat of Example 7, was preparedexceptthat-the ratio of structural fiber/binder/pore-former was .10/10/80.The. extr ct p u h a le her permeability of*23,7;21 and a tongue-tearstrength of 0. Pounds.

Example 14 T016300 parts of water containing 0.2 part of alkyl 'arylpolyglycol ether were added, with stirring, 4.00parts of 0.5"", 3denier/filament polyhexamethylene adipamide structural staple and 5.34parts of 0.5", 25 denier/ filament polyvinyl alcohol pore-former fiber.The fiber dispersion was formed into a mat by filtration through an8-rnesh wire screen. The mat was impregnated with 40 parts of a solutionof polyvinyl chloride/di- .(2-ethylhexyl)tphthalate /60 disperse-d at10% concentration in tetrahydrofurane/dimethyl formarnide 9.75/25. Afterdrying, the impregnatedv mat was pressed for 10 minutes at C. and 1,245lbs./sq. in.; and the polyvinyl alcohol. pore-former was finallyextracted with 90 Curunning water. Complete extraction required 96hours. The product had a leather permeability of. 3,805. The ratio ofstructural fiber/binder/pore-former was 30/30/40.

Example 15 TABLE I Percent Percent Leather Permeability Pore- PoreandLength of Example Fotpruer Former .Pore-Forming-Eiher y y Volume Weightv 0.01" Flock 10.5 Staple .leatherpermeability.

Example 16 To 16,000 parts of water containing 0.2 part of octyl amidestaple and 4.125 parts of 0.01", 3 denier/filament cellulose acetatestaple. The fiber dispersion wasformed into a that by filtration throughan S-rriesh wire screen of area such as to provide a fiber weight of 2.3oz./ sq. ft. The mat was impregnated with 12.05 parts of a 14% latex ofN-met-hoxymethyl polyhexamethylene adipamide (DV=55), dried, pressed forfive minutes at 140 C. and 1,930 lbs/sq. in. (structuralfiber/binder/poreformer ratio of 22.5/22.5/S); and the cellulose acetatewas finally extracted with warm running acetone.

The product had a leather permeability of 5,530 and a tongue-tearstrength of 2.3 pounds. By comparison with the product described inExample 1, it will be seen that this increase in length of thestructural fiber resulted in a corresponding increase in the tearresistance of the Example 17 A product was prepared as in Example 16except that 0.5" polyhexamethylene adipamide staple was replaced with a.corresponding amount of 0.01", 3 denier/filament polyhexamethyleneadipamide staple; and the 0.01" cellulose acetate staple was replacedwith a corresponding amount' of 0.25", 1 denier/filament celluloseacetate staple. The product exhibited a leather permeability of 13,452and a resistance to water liquid penetration of 172 flexes.

Example 18 The product was prepared in a manner similar to that ofExample 16. In this example, 1.5 parts of 0.5" 3 denier/filamentpolyhexamethylene adipamide staple; 4.5

parts of 0.5, 3 denier/filament cellulose acetate staple;

and 10.7 parts of a 14% latex of N-methoxymethyl polyhexamethyleneadipamide were combined to give a structural fiber/binder/pore-formerratio of 20/20/60. The product had a leather permeability of 13,706 anda tongue tear strength of 3.5 pounds.

A comparison of the permeabil-ities exhibited by the products ofExamples 16--18 inclusive indicates that the permeability increased asthe pore-forming fiber length was increased up to about 0.25", beyondwhich no further increase in permeability was obtained by increasing thepore-forming fiber lehgth, even when the pore-forming content was raisedfrom 55 to 60%.

. Although little additional strength can be gained by increasing thelength of the structural fiber beyond 1.5" and little additional abilityto breathe can be gained by "increasing the pore-former fiber lengthbeyond 0.25, it

may be convenient to use longer fibers. Various standard types oftextile machinery which may be used in thepro' duction' of thesesynthetic leather compositions are designed to handle staple fibers inlimited length ranges which may lie anywhere in the neighborhood of 0.5"to as long as 8''.

for these compositions using even longer fibers.

The following examples-illustrate the effect of fiber denier, i; e.,both structural and power-forming fiber,

It is also possible to make usable mats upon leather permeability.

Example 19 v I I Aproduct was prepared as in Example 16 except that p0.375", 1 denier/filament polyhexamethylene adipamide staple and 0.01",1 denier/filament cellulose acetate poreformer were used. Afterextraction of the cellulose acetate pore-former, the product had aleather permeability of 3,992 and a water liquid penetration resistanceof 1,227 flexes.

Example 20 A product was prepared as in Example 19 except that 0.01",1.5 denier/filament cellulose acetate staple was employed. Afterextraction of the pore-former, the product had a leather permeability of3,940 and a water liquid penetration resistance of 171 flexes.

Example 21 A product was prepared as in Example 19 except that 0.01", 3denier/filament cellulose acetate staple was employed as thepore-former. After extraction of the poreformer, the product had aleather permeability of 4,888 and a water liquid penetration resistanceof 80 flexes.

Example 22 A product was prepared as in Example 19 except that 0.5, 3denier/filament polyhexamethylene adipamide staple and 0.01", ldenier/filament cellulose. acetate staple were employed as structuralfiber and pore-former, respectively. After extraction of thepore-former, the product had a leather permeability of 4,126 and a waterliquid penetration resistance of 238 'fiexes.

Example 23 A product was prepared as in Example 19 except that 0.5", 1denier/filament polyhexamethylene adipamide staple and 0.01, 1denier/filament cellulose acetate were employed as structural fiber andpore-former, respectively. The extracted product had a leatherpermeability of 3,836.

' Example 24 A product was prepared as in Example 19 except that 0.5", 3denier/filament polyhexamethylene adipamide staple and 0.01", 1.5denier/ filament cellulose acetate were employed as structural fiber andpore-former, respectively. The extracted product had a leatherpermeability of 4,649 and a water liquid penetration resistance offlexes.

The results of Examples 19-24, inclusive, indicate that variations instructural fiber and pore-forming fiber thick- .ness result in littlevariation in the leather permeability of the synthetic leathercompositions. Generally, increasing the thickness of the pore-formingfiber decreases the .re-

sistance to liquid water penetration.

Fibers which are finer than 1 denier are difficult to process onstandard textile machinery; and when used as structural fibers in thepresent compositions, the fibers are too rigidly immobilized by thebinder to give optimum strength, e. g., tear and tensile. In otherwords, the fibers of greater denier tend to shift within the binderunder flexing or tearing, this action making the product more durable.Finer fibers, however, are rigidly held by the binder. Furthermore,fibers having a denier less than /2 have so much exposed surface, i. e.,ratio of surface area to core volume is high, and are so soft that theyno longer behave as normal textle staples. On the other hand, fiberscoarser than 16 denier are extremely harsh and more closely resemblebristles.

The following Examples 25-26 illustrate the effects of thickness of theextracted sheets upon the value of leather permeability.

Example 25 A product was prepared in a manner similar to that describedin Example 1 except that the thickness of the extracted Product was0.0197, and its weightwas 0.67 (gm/sq. ft. The product had a leatherpermeability of Example 26 A product was prepared in a manner similar tothat of Example 25 except that the, thickness of the resulting extractedproduct was 0.071". The leather permeability was 4,281.

The above examples indicate thatpermeability via in! ternal surfacediffusion is only halved by an approximate 4-fold increase in thethickness. This is in contrast to the fact that with homogeneous films,the relationship 'between thickness and permeability is essentially"linear for relatively hydrophobic compositions, but varies fromlinearity with hydrophilicfilms wherein an increase in film thicknessdoes not give the calculated decrease in permeability.

The following examples illustrate the use of various types of syntheticpolymers as binders and the incorporation of such binder polymers intothe compositions in various forms, e. g., as homogeneous films, fromsolvent solutions, from aqueous dispersions, etc.

Example 27 A mat consisting of 1.688 :partsof 0.01, 3 denier/filamentpolyhexamethylene adipamide staple and 4125 parts of 0.01", 3denier/filament cellulose acetate staple was prepared from dispersion in600 parts of water by filtration through a. filter with an area such asto provide a fiber Weight of 2.3 oz./sq. ft. The mat was dried,preheated for 15 minutes at 180 C. in an atmosphereof nitrogen, and thenimpregnated with 1.688 parts of a film f N-methoxymethylpolyhexamethylene adipamide (DV=86) by pressing for 10 minutes. at 160C. and .1450 lbs/sq. in. The product had a structural fiber/binder/pore-former ratio of 22.5/22.5/5.5. After extractionof thecellulose acetate, the leather permeability was 7,294; the productresisted 48 flexes before permitting water liquid penetration; and itresisted 213,000 flexes in the Schiltknecht machine before surfacecracking developed.

Example 28 A mat was prepared in a manner similar to Example 16 using0.5" cellulose acetate staple and replacing the latex with 29.6 parts ofa dispersion of ,N-methoxymethyl polyhexamethylene adipamide (DI/=86) at5.7% concentration in ethanol/ water 80/20. After drying, theimpregnatedmat was pressed for 15 .minutes at 165 C. and 1245 lbs./ sq.in; and the cellulose acetate pore-former was finally extracted withacetone. The tough, permeable, pliable, leather-like-product was 0.028thick and. Weighed 1.48 oz./sq. ft. It had a leather permeability of13,778, a tensile strength of 3490 lbs/sq. in., an elongation of 38%, amodulus of 13,652 lbs/sq. in., and a tongue-tear strength of 16.0pounds.

Example 29 Applied non-woven fibrous mat was prepared in a mannersimilar to Example 3. This composition was composed of a top layer of 1part of a copolyester of ethylene glycol, 60 mol percent of terephthalicacid, and 40 mol percent of sebacic acid as the binder; and 1 part ofcellulose acetate fiber, 3 denier per filament, 1.5" in length. Anintermediate layer was composed of 3 parts of the copolyester binder, 3parts of polyethylene terephthalate structural fiber, and 9 parts ofcellulose acetate poreeforming fiber. A base layer was composed of partsof the copolyester binder, 5 parts "of polyhexamethylene adipamidestructural fiber. and 15 parts .of cellulose'acetate fiber'pore-former.The layers were composited under heat and pressure, and the celluloseacetate was extracted. The resulting product C10 weigh d 13 czJsq..-,ft., ha a leather permeation: 13, and a .Schiltkuecht .fiex life of553.000 flexes.

Example 30 A product was prepared as in Example 27 except that thepolyamide film was repl ced with an eq al. amoun of polyethylene. film,and the pressing waseondueted for 5 minutes at 175 C. and. 3390 lbs/sq.in, after preheating the mat 15 minutes at 180 C. in a nitrogenatmosphere. After extraction of the cellulose acetate, the product had aleather permeability of 8,987, a water liquid penetration resistance of11,000 flexes, and a Schiltknecht flex life of 6,000. It will be notedthat this composition ;was superior to natural shoe leather such asScotch Grain Cowhide and Cretan Butt in the incompatible properties ofhigh leather permeability and low water liquid permeability.

Example 31 A mat comprising 1.688 parts of .001", 3 deni'er/ filamentpolyhexamethylene adipamide staple and 4125 parts of 0.01, 2.5denier/filament polyvinyl alcohol staple was prepared by the proceduredescribed in Example 27. The mat was preheated 15 minutes at 180 C. inan atmosphere of nitrogen and then impregnated with 1.688 grams of afilm comprising a polyvinyl butyral. resin/dibutyl sebacate composition70/30 by pressing for 5 minutes at 155 C. at 1,450 lbs/sq. in. Thepolyvinyl alcohol pore-former was then extracted with hot running water.The extracted product had a leather permeability of 3,719, a waterliquid resistance of 748 flexes, and a Schiltknecht flex life of 47,500.

Example 32 A product was prepared as in Example 31 except. that thepolyvinyl acetal film was replaced with an equal amount 'of a filmcomprising N-methoxymethyl polyhex'ame'thylene adipamide (DV=55)/met hylIO-phenol stearate 50/50. This film was impregnated into thepolyhexamethylene adipamide/polyviny'l alcohol fiber mat by preheatingthe mat for 15 minutes at 180 C. in an atmosphere of nitrogen, followedby pressing for 5 minutes at 160 C. and 1,450 lbs/sq. in. pressure. The.product, after extraction of the polyvinyl alcohol, had a leatherpermeability of 12,428 and exhibited sure face cracking after 78,000Schiltknecht .flexes.

Example 33 A mat Was prepared at a Weight of 3.3 oz./sq. ft. asdescribed ,in Example 16, using 4.68 parts of 0.5", 3 denier/filamentpolyhexamethylene adipamide structural fiber and 11.46 parts of 0.5",2.5 denier/filament polyvinyl alcohol pore-formingfib'er dispersed in16,000 parts of water contain ng 0.2 .part of c yl ph ny p y y ether.After drying, themat as imp g it 65 parts of a dispersion at 7.2%concentration in methyl ethyl ketone of vinyl chloride/vinyl acetate /5copo yrner/di-(2- hyly phtha at /60. The freshly impregnated mat wassubmerged in water, dried, and pressed for 10 minutes at C. and 1245lbs/sq. in.; and the polyvinyl alcohol pore-former was finally extractedwith running water at 90 .C. The extracted mat was split .down themiddle to givea very leather-like product with a fiber-rich flesh sideand a binder-rich grain side. The split product was 0.027" thick andweighed 1.00 oz./sq. it. It .had a leather permeability of 12,058, atensile strength of 2,025 lbs/sq. in., an elongation of 48%, a modulusof 14,983 lbs/sq. in., a tongue-tear strength of 6.3 lbs., and aSchiltknecht flex life .of 285,000.

- Example 34 A mat was prepared in a manner. similar to that 2.0:

Example .33 replacing theplfasticiged vinyl chloride/vinyl acetatecopolymer with an equivalent amount of polyvinyl chloride/di-(2-ethylhexyl) phthalate 100/60, applied from dispersion at 6.2% concentrationin tetrahydrofurane/dimethyl formamide 98/2. The impregnated mat wassubmerged in water, dried, pressed for minutes at 160 C. and 1245 lbs./sq. in. using 0.030 shims; and the pore-former was extracted withrunning water at 90 C. The leather-like product was 0.031" thick andweighed 1.51 oz./sq. ft. It had a leather permeability of 7,908 and atongue-tear strength of 4.1-7.1 pounds.

Example 35 A product was prepared as described in Example 16 except thatthe binder was composed of chlorosulfonated polyethylene/woodrosin/tribasic lead maleate/titanium dioxide/aluminum stearate/Captex(Z-mercaptobenzothiazole)/diphenyl guanidine 100/ 10/20/ 20/ 2/ 3/0.5,dispersed at 7.6% concentration in toluene. The freshly impregnated matwas submerged in methanol, dried, pressed for 10 minutes at 170 C. and1,245 lbs./sq. in. using 0.030" shims; and the cellulose acetateporeformer was finally extracted with acetone. The leatherlike productwas 0.028" thick and weighed 1.43 oz./ sq. ft. It had a leatherpermeability of 16,551 and a tonguetear strength of 8.0 pounds.

Example 36 A product was prepared in a manner similar to Example 28except that the binder was polyethylene, and it was applied from a latexat 4.5% concentration. After drying, the impregnated mat was pressed for10 minutes at 175 C. and 1,245 lbs./sq. in. using 0.030" shims; and thecellulose acetate pore-former was finally extracted with acetone. Theleather-like product was 0.029" thick and weighed 1.48 oz./sq. ft. Ithad a leather permeability of 14,216 and a tongue-tear strength of 5.2pounds.

Example 37 A mat comprising 1.688 parts of 0.01", 3 denier/filamentpolyhexamethylene adipamide staple and 4.125 parts of 0.01, 3denier/filament acetate staple was preheated minutes at 65 C. andimpregnated with 16.3 parts of a hot 10.34% solution in toluene ofchlorinated polyethylene of 27.5% chlorine content. The solvent wasallowed to evaporate, and the impregnated mat was pressed 10 minutes at190 C. and 1,450 lbs./sq. in. After extraction of the cellulose acetatepore-former, the product had a leather permeability of 3,531, a waterliquid resistance of 21,584 flexes, and a Schiltknecht flex life of8,000.

Example 38 A mat was prepared in a manner similar to that described inExample 28; and the ethylene/vinyl acetate 2.36/1 copolymer binder,dispersed at 7.2% concentration in toluene, was employed. After drying,the impregnated mat was pressed for 10 minutes at 180 C. and 1245lbs./sq. in. using 0.030" shims. After extraction of the celluloseacetate pore-former, the product was 0.026" thick and weighed 0.9oz./sq. ft. The product had a leather permeability of 19,430, aresistance to liquid water penetration of 18,00024,000 flexes, and atongue-tear strength of 15 pounds.

Example 39 A mat comprising 1.688 parts of 0.01" 3 denier/filamentpolyhexamethylene adipamide staple and 4.125 parts of 0.01", 2.5denier/filament polyvinyl alcohol staple was prepared by the proceduredescribed in Example 27.

.This was impregnated with 26.65 parts of 6.33% latex of .a vinylidenechloride-acrylonitrile copolymer/dioctyl Example 40 A mat prepared asdescribed in Example 28 was imprgenated with a dispersion of neoprene(poly-2-chloro- 1,3-butadiene) zinc oxide/ magnesiumoxide/phenyl-betanaphthylamine /10/10/2 at 7.2% concentration in benzeneto give a structural fiber/binder/pore-former ratio of 22.5/ 22.5/ 55.The freshly impregnated mat was submerged in methanol, dried, pressedfor 40 minutes at C. and at 1,245 lbs./sq. in., using 0.030 shims; andfinally the cellulose acetate pore-former was extracted with acetone.The weight of the extracted product was 1.41 oz./ft., and its thicknesswas 0.028". It had a leather permeability of 8,638, a water liquidpenetration resistance of flexes and a tongue-tear strength of 10pounds.

The following example illustrates use of the synthetic leathercompositions of the present invention in the fabrication of shoe uppers.

Example 41 In order to fabricate shoes from the synthetic leathercompositions of the present invention, composite sheets consisting offibers of polyhexamethylene adipamide, cellulose acetate, and acopolyester made from ethylene glycol and 60 mol percent of terephthalicacid and 40 mol percent of sebacic acid were prepared. In general, theinitial non-woven water vapor-impermeable fiber sheets were made bycarding the mixed fibers together and then hot-pressing at 500 lbs/sq.in. at C. the carded fibrous mat to form a composite sheet. In allcases, the fibers were 2.5" in length and 3 denier/ filament.

To prepare the non-woven fibrous impermeable sheets, the three fibrouscomponents were blended on a Garnet card and were collected in the formof a hat or web of the desired thickness by wrapping the primary thinweb from a doffer comb around a collecting drum a suitable numberofrevolutions. As indicated in Table II, some of the mats were formedfrom two webs of the same composition; and some of the mats werecomposed of two different webs. In one of the mats, an additionalquantity of binder polymer was introduced in the upper layer of the matby assembling a homogeneous film on top of the two webs. It should benoted that this did not remain in the form of a film in the finalproduct, but was pressed entirely into the fibers and existed merely asan additional binder in the composition. The complete assembly of webswith or without film was laminated between cellophane and subjected toheat and pressure to melt the binder and make it flow into the fibrouswebs. All of the compositions were cooled under pressure and extractedat room temperature in acetone. In Table II, the samples coded Ae-Eehave the same composition as samples A-E except that the initial fibrousimpermeable mats were embossed with heat and pressure at a slightlylower temperature than that at which they were made, the embossing beingcarried out before extraction of the cellulose acetate fibers. Thisprocedure represents an outstanding advantage of the process of thepresent invention whereby a synthetic leather composition with anembossed surface may be prepared by carrying out the embossing stepprior to extraction of the pore-forming fiber. Obviously, embossingafter extraction compresses the capillary structure of the compositionsand decreases the water vapor permeability.

It will be noted that leather permeability values in the following tablewere measured at 23 C. and 80% relative 'humidity (R. H.), and this onlyapplies to the examples in this table. v

TABLE II Wright Ratio of Structural L. P. V. Fiber/Binder/Pore-FormerGrams/100 Tongue- Thick- Elonga- Oode sq. meters Tear, ness Tenacitytlon Modulus hr. at 80% Pounds (inches) (I). s. i.) (percent) (p.s. i.)Nylon T/10 2 Cellulose R. H; and

Acetate 23 C.

1 l 3 13, 088 24. 3 031 1, 839 93. 3 11.377 1 1 2 4, 818 18. 4 031 3,205 102 22, 153 1 1 4, 13, 522 16. 8 045 1, 508 109. 5, 771 Three-LayerComposition 1 Mil Continuous T/lO 45/55 4 1 1 3 14, 348 18.0 032 1, 804100 8, 412 (Embossed Surface) 1 1| 2 8, 839 21.0 026 1, 776 86. l 12,786 (Embossed Surface) Three-Layer Composition Top. 1 Mil of ContinuousT/10 45/55 Ee Mid l 1 3 11, 650 029 1, 490 100 7,1453

Bot. 1 l

(Embossed SuIrface) 1 Polyhexamethylene adipamide.

Z Copolyester of ethylene glycol and 60 mol percent terephthalic acidand 40 mol' percent scbacic acid. 3 Copolyester of terephthalic andsebacic acids in the stated ratioywith ethylene glycol.

structural fibers of I greater length, e. g., 2-2.5", and show thesuperior tear strength of the resulting synthetic leather compositionsas compared to various types of genuine leather.

Examples 4249;are presented in tabularform in Table III, the preparationof these synthetic leather compositions being carried out in accordancewith the general procedure: described in Example 41. It will be notedthat The following examples mainly illustrate the use of 35 tearstrengths of the compositions in the following table TABLE III WeightRatio Structural Fiber Binder Example Denier Length StructuralFiber/Binder/Pore-Former Nature Fila; (Inches) Nature 1 Used As menControl Brown topgrain (upholstery) cowhlde... Do Red Morrocco finishtopgrain upholstery cowhide. Do Yellow topgrain heifer hide (shoeupper)., Do High quality cotton duck (army tents,

tarpaulins) 18.5/ Polyethylene film 12.5/12 /75 T-.10**- Staple fiber(1.5, 7 den./ fila.) 3 22.5 T* J10 15 2-2.5 'I10** -d0 1. 5 1. 5 T-10"-do /20/60 top film hlor fonated poly- 3 2. 5 T-10 film w h top filmethylene as additional binder. of chlorosultonated polyethylene (0002)27/18/55 Polyethylene Ter- 3-4 2 Polyethylene film ephthalate. /24/51.do 3-4 2 do .do

Pore-Forming Fiber Thickness Trape- Percent Wt. of of Tenacity, zoidalTongue- Elonga- Example Extracted Extracted #linJoz. Tear Tear LPV,g./100 tlon Denier Length Product Product yd. 2 Strength, Strength, sq.meters/hr. At Nature File; 1 (Inches) oz./yd. (Inches) #/oz./yd.#/0Z./yd- 2 Break men nylon=polyhexamcthylene adiparnidc. I v lTl0=copolyestcr of ethylene glycol and 60 mol percent terephthallc acidand mol percent sebacic acid.

are expressed in terms of trapezoidal tear strength in lbs./oz./sq. yd.This test is described in Federal Specification CCC-T-191a dated October5, 1945. For the sake of comparison, the trapezoidal and tongue-tearstrengths (both expressed in lbs./oz./sq. yd.) are given for threedifferent types of genuine leather. For these samples, the ratio oftrapezoidal to tongue-tear strength ranges from 3 to 7. Hence, thereappears to be no constant conversion factor.

It is to be understood that the foregoing examples are merelyillustrative and that the present invention broadly comprises forming acompact, essentially impermeable and continuous, composite sheet byhot-pressing a composition comprising a structural fiber component, anextractable pore-forming fiber component, and a binder material, theproportion of pore-forming fiber, based upon the total volume of thesheet, being from 40-70%, the weight ratio of structural fiber to bindermaterial being from 1/2 to 2/ 1, and thereafter forming interconnectingpores having essentially the shape of the poreforming fibers in thesheet by extracting the pore-forming fiber.

Fibers of nylon, i. e., synthetic linear polyamides such aspolyhexamethylene adipamide, polyhexamethylene sebacamide,polycaproamide and interpolyamides, etc., are outstanding for use as thestructural fibers in the synthetic leather compositions of the presentinvention. The use of nylon structural fibers produces an extractedcomposition having high tear strength, tensile strength, softness andfiex life. Polyethylene terephthalate homopolymer and copolymer fibersare also considered to be good structural fibers; and other natural andsynthetic fibers which may be employed include polyacrylonitrile,acrylonitrile copolymers, cellulose acetate, viscose, polyvinyl acetals,cotton, wool and glass fibers. The length of the structural fiber may bevaried depending upon the general strength properties required. Asillustrated in the foregoing examples, structural fibers as short as0.01 may be employed; but structural fibers having a length of about1.5" give a product of substantially optimum strength properties. Usingstructural fibers longer than 1.5" imparts little additional strength tothe present synthetic leather compositions; but, as mentionedhereinbefore, it may be convenient to use longer fibers; and the use oflonger structural fibers, e. g., up to 8", is within the intended scopeof the present invention. On the other and it should besubstantiallyinert, i. e., have no solvent action, toward the structuralfiber and the binder material. The duration of the extraction stepdepends upon the weight of the pore-forming fiber in the initial sheet,the thickness of the sheet, the solubility of the pore-forming fiber inthe solvent, the temperature of the extracting liquid, and the degree ofagitation.

As illustrated in the foregoing examples, products of satisfactoryleather permeability may be formed using hand, structural fibers lessthan 0.01" in length add little to the strength of the sheet over thatof a sheet composed wholly of the binder polymer.

It should be emphasized that the structural fiber component should notbe excessively softened at the flow temperature of the binder materialand should be insoluble in the solvent used to extract the pore-formingfiber.

The pore-forming fiber must be a workable staple fiber; that is, itshould be adaptable to making bats. In this form, the pore-forming fiberis randomly disposed; and

extraction thereof results in the formation of a network ofinterconnecting capillaries or pores. Furthermore, the pore-formingfiber must not be excessively softened at the flow temperature of thebinder and must be readily pore-forming fibers as short as 0.01, e. g.,0.01" cellulose acetate flock. However, a considerable increase inleather permeability is realized when longer pore-forming fibers areemployed. For example, increasing the length of the pore-forming fiberfrom 0.01" (Example 16) up to a length of 0.25" (Example 17) produces asubstantial increase in the leather permeability of the resultingproduct. However, as illustrated by comparing Examples 17 and 18, anincrease in pore-forming fiber length from 0.25" to 0.5" resulted insubstantially no increase in the leather permeability of the resultingsheet.

As illustrated in Examples 19-24, variations in the denier of thestructural fibers and pore-forming fibers appear to have little effectupon leather permeability. On the other hand, it has been found thatdecreasing the thickness of the pore-forming fiber results in theformation of a synthetic leather sheet having improved re sistance topenetration by liquid water. On the other hand, fibers which are finerthan 1 denier are difiicult to process on standard textile machinery;and, when used as structural fibers in the present compositions, thefibers are too rigidly immobilized by the binder to give optimumstrength, e. g., tear and tensile. In other words, the fibers of greaterdenier tend to shift within the binder under flexing or tearing, thisaction making the product more durable. Finer fibers, however, arerigidly held by the binder. Furthermore, fibers having a denier lessthan /2 have so much exposed surface, i. e., ratio of surface area tocore volume is high, and are so soft that they no longer behave asnormal textile staples. On the other hand, fibers coarser than 16 denierare extremely harsh and more closely resemble bristles.

It is in the binder material that the interconnecting capillaries orpores are formed upon extraction of the pore-forming fibers, theinterconnecting capillaries or pores having the shape of staple fibers.Furthermore, the binder material holds together and is reinforced by thestructural fibers. The binder material may be selected from a greatvariety of soft, elastic, initially thermoplastic, synthetic polymerswhich may be classified generally as elastomers, as set forth by H. L.Fisher (Industrial and Engineering Chem, August 1939, page extractablefrom the composite sheet with a solvent which is substantially inert, i.e., has no solvent action, to the structural fiber and the bindermaterial. As illustrated in the foregoing examples, cellulose acetate isreadily extractable using acetone and some grades of polyvinyl alcoholas readily extractable with water. Other suitable pore-forming fibersare those of sodium alginate, potassium metaphosphate polymer glass, andcarboxymethyl cellulose.

The choice of a solvent for extracting the pore-forming fiber from theinitial impermeable sheet depends upon the particular pore-forming fiberand the nature of the structural fiber and the binder material. Thesolvent should be one in which the pore-forming fiber is readilysoluble;

942). The following polymers are preferred: N-methoxymethylpolyhexamethylene adipamide, copolyesters made from ethylene glycol,terephthalic acid and sebacic acid of the general types disclosed andclaimed in copending applications U. S. Serial Nos. 150,811 and 150,812,filed March 20, 1950, in the name of M. D. Snyder, polyethylene and itsderivatives, plasticized polyvinyl chloride, plasticized vinylchloride/vinyl acetate copolymers, natural rubbers, synthetic rubberssuch as neoprene (2- chloro-1,3-butadiene polymer) and various othercompositions containing'vinyl chloride polymers or copolymers. Variousother specific synthetic linear polymers which may be employed as abinder material with or without plasticizers include polyvinyl acetals,such as polyvinyl butyral or laural, chlorinated polyethylene,chlorosulfonated polyethylene, ethylene/vinyl acetate copolymers,vinylidene chloride/acrylonitrile copolymers, polyethyleneterephthalate, etc., or any tough, pliable poly merie material which isat least initially thermoplastic and which melts or flows at atemperature below the deformation (softening) temperatures of thestructural fiber and pore-forming fiber. By the term initiallythermoplastic is meant that the binder material must melt and flow underthe conditions of the hot-pressing 17 step. When the binder material isinthe form of individual fibers, the length of the fiber has no effectupon the properties of the extracted sheet, provided that the binderfibers have been uniformly dispersed throughout the composition beforeheat and pressure are applied.

In describing the binder material as a tough, pliable, at leastinitially thermoplastic, polymer, the following more specificrequirements apply to those binder materials that are preferred:

1. The tensile strength should be at least 500 p. s. i.

2. The elongation must be at least 100%.

3. Materialsnot having a tensile strength and elongation greater thanthe above minimum specifications are satisfactory if the product oftheir tensile strength and elongation (where 100%:1) is at least 1,000.

4. The modulus must not be more than 25,000 p. s. i. and, preferably,not more than 5,000 p. s. i.

The binder material may be incorporated into the initial impermeablecomposite sheet in a variety of ways, several of which are illustratedin the foregoing examples. As illustrated in Example 2, the bindermaterial may be in the form of fibers which may be carded along with thestructural fiber and the pore-forming fiber to form a composite sheet bypressing the carded mixed fibers at elevated temperatures. Examples 1and '7 illustrate mutual coagulation of a mixed dispersion of structuralfibers, pore-forming fibers and binder polymers. Example 16 illustratesformation of a fibrous mat of a mixture of structural and pore-formingfibers followed by impregnation of the mat with a binder polymer in anaqueous dispersion. Other techniques of incorporating the bindermaterial include impregnation of a fibrous mat with a binder polymer insolvent solution and impregnation of a fibrous mat with a binder polymerin the form of a powder or a homogeneous sheet. Other techniques ofincorporating a binder material with the fibrous components (structuraland pore-forming fibers) of the initial sheet include hot meltimpregnation, impregnation by calendering or by spraying the bindermaterial from aqueous dispersion or solvent solution onto one or bothsides of a fibrous mat, followed by heat and pressure to impregnate thefibrous portion of the sheet with the binder material. Regardless of thetechnique employed to form a composite sheet, the sheet is consolidatedprior to extraction by pressing at a temperature which is above the fiowtemperature of the binder and below the softening temperature of thestructural fiber and pore-forming fiber. The pressure used is sufiicientto cause the binder to flow and thoroughly impregnate the fibrouscomponents of the sheet.

On the basis of numerous types of actual wearing comfort tests, it hasbeen ascertained that membranes, e. g., leather and synthetic leather,exhibiting a leather permeability of 2,000 to 10,000 gms./ 100 sq.meters/hr. would provide wearing comfort equivalent to that obtainedwith glazed leathers and heavy shoe upper leathers. Furthermore,permeabilities of 10,000 to 22,000 were found to provide adequatewearing comfort equivalent to that obtained with the lightest shoeleathers.

In general, the wearing comfort of boots, shoes, gloves, etc., isdetermined or evaluated by measuring the leather permeability of thematerial from which the article is fabricated. With synthetic leathercompositions made by the process of this invention, the leatherpermeability is substantially directly dependent upon the proportion ofpore-forming fibers in the initial impermeable sheet. It is to beunderstood that the initial sheet, i. e., before extraction of thepore-forming fibers, is substantially continuous and vapor-impermeable.As mentioned hereinbefore, generally satisfactory comfort for boots andshoes is obtained when the shoe upper material has a leatherpermeability of at least 2,000 gms./100 sq. meters/hr. in an atmosphereof 23 C. and 90% R. H. Expressed in terms of one of the criticallimitations of the present invention, it has been found that the totalvolume of voids, i. e., interconnecting pores produced by extraction ofthe-pore-forming fiber, should be no less than 40% of the total volumeof the poroussheet to give at least satis factory wearing comfort.Hence, since the densities of the components, i. e., structural fiber,binder material and pore-forming fiber, of the composite aresubstantially the same, this means that the proportion of thepore-forming fiber should be at least 40% of the total volume of theinitial composite sheet. On the other hand, the foregoing examplesclearly illustrate that the strength of the synthetic leathercompositions of this invention drops off appreciably when the volume ofthe pore-forming fiber in the initial unextracted tructure issubstantially greater than 70% of the whole. This means that thestrength of the synthetic leather compositions of the present inventionis substantially below optimum when the total volume of voids in thefinal composition is greater than 70% of the whole. Resistance topenetration to liquid water is also unduly decreased at greater than 70%poreformer. The optimum volume of voids or pores in the finalcomposition appears to be from 5060%. As illustrated in the foregoingexamples, the value of leather permeability increases as the totalvolume of voids increases; but the amount of void space, is limited bythe ultimate strength desired. For example, kid leather has a tonguetearstrength of only 3.15 pounds; and coated fabrics, for example, a wovencotton fabric coated with N-methoxymethyl polyhexamethylene adipamide,which have been successfully fabricated into shoes, have a tongue-tearstrength of 2.62 pounds. On this basis, the very minimum tongue-tearstrength of synthetic leather compositions is in the neighborhood of onepound (tongue-tear) although at least a tongue-tear of 3 to 5 pounds ispreferred.

As mentioned hereinbefore, the prevailing problem in the production of asynthetic leather having satisfactory wearing comfort is that ofpreparing a sheet material of satisfactory tear strength, tenacity, flexlife, softness, coupled with the ability to transpire water vapor andair. In addition to the direct effect of pore-former content uponleather permeability, it has been found that the weight ratio ofstructural fibers to binder material is also a critical factor in thepreparation of a comfortable material of satisfactory strength. Asillustrated in Examples 1, 5 and 6, optimum tear strength is obtainedwhen the weight ratio of structural fibers to hinder material is betweenl/ 2 and 2/1. Synthetic leather compositions of this invention in whichthe weight ratio of structural fibers to binder material is outside thisrange possess tear strengths substantially below the optimum. This issurprising and is not predictable on the basisof prior art on thepreparation of impermeable non-woven sheets.

In addition to possessing optimum tear strength when the ratio ofstructural fibers to binder fibers is within the above specified limits,it is surprising that the present synthetic leather compositions aremore durable in general service than conventional coated fabrics madefrom the same fiber and binder, i. e., film-former, material. Forexample, a woven nylon fabric coated with a typical binder polymer ofthis invention does not possess the unique combination of tear strength,tensile strength, softness, and flex life, which are characteristic ofthe synthetic leather compositions of this invention. Furthermore, thecoated fabrics are substantially impermeable to air and water vapor. Inaddition, the formation of a synthetic leather composition by extractionof a poreformer fiber results in producing a composition which isgenerally superior to all types of known synthetic leathers, especiallywith respect to the combination of tear strength, flex life, tensilestrength, pliability and water vapor and air permeability. In general,the present synthetic leather compositions surpass all previous leathersubstitutes and, frequently, even natural leather itself.

The process of the present invention is exceptionally versatile withrespect to preparing a synthetic leather composition of the desiredinternal structure and surface texture. As mentioned hereinbefore,embossed sheets may be readily and efliciently produced by employingembossing rolls or pressure-applying surfaces during composition andfusing of the combination of structural fibers, binder and pore-formingfibers. Hence, embossing may be carried out in conjunction with anecessary step in the process; or it may be carried out immediatelythereafter under conditions of lower temperature and pressure thanemployed in the fusing step. Embossing the extracted composition resultsin substantially decreasing the permeability of the structure and isgenerally to be avoided.

Another outstanding advantage of the present process is that thecross-sectional structure of the present synthetic compositions may betailored for the desired end use by laminating or fusing different pliesof one or more of three basic components together, the binder componentusually being in fiber, powder or film form. On the other hand, initialfibrous mats composed of a mixture of the structural and pore-formingfibers may be carded and thereafter impregnated with the binder materialby any of the methods mentioned hereinbefore. Examples 2, 3 and 41illustrate laminating various plies containing one or more of the threebasic components together in order to control the face-to-backconcentration of structural fiber, binder material and pore-former. Forexample, the top ply of the initial unextracted composition may becomposed entirely of the binder polymer in the form of a homogeneousfilm. The middle ply may be composed of various amounts of thestructural, binder and pore-forming fiber; and the bottom ply may becomposed essentially of the structural fiber. Such a structure afterlamination and extraction of pore-former would form a synthetic leathercomposition having a relatively smooth or non-fiber-like upper surface(usually referred to as the skin side); and the bottom side would besubstantially fibrous (usually referred to as the flesh side). It isobvious that the concentration of the structural fiber, binder materialand pore-former may be varied from the face to the back of the sheetwithin reasonable limits by empolying the general practices of thepresent invention.

An additional advantage of the present invention is that it provides asynthetic leather having a unique combination of tear strength, tensilestrength, softness, pliability, fiex-lifeand ability to transpire watervapor and air. A further advantage is that it provides a process ofpreparing such synthetic leather compositions especially useful infabricating boots, shoes, gloves, etc., wherein the properties may betailored to the desired end use. A still further advantage is that itprovides an ecomonical process of preparing a synthetic leather which isequal or superior to the various types of genuine leather,

As many widely different embodiments'may be madewithout departing fromthe spirit and scope of this invention, it is to be understood that saidinvention is -in no way restricted except as set forth in the appendedclaims.

We claim:

1. The process of forming non-woven porous fibrous sheet which comprisesforming a non-woven fibrous sheet comprising essentially structuralfibers from the group consisting of staple fibers of synthetic linearpolyamide, polyethylene terephthalate, polymers of acrylonitrile,polyvinyl acetals, regenerated cellulose, cotton and wool, from 40% toby volume based on the total volume of the sheet, of pore-forming fibersselected from the group consisting of polyvinyl-alcohol, celluloseacetate, sodium alginate and carboxymethylcellulose, and a softelastomeric binder material having a flow temperature below thedeformation temperature of said fibers, the Weight ratio of structuralfibers to. binder material being within the range of from 1:2 to 2: 1,hot pressing said sheet at a temperature above the flow temperature ofthe binder and below the deformation temperature of said fibers, andthereafter extracting said pore-forming fibers from the sheet with aliquid which is a solvent for said pore-forming fibers and a non-solventfor said structural fibers and binder material whereby said structuralfibers are substantially uniformly distributed throughout said poroussheet.

2. The process of claim 1 in which the structural fibers are a syntheticlinear polyamide.

3. The process of claim 2 wherein the length of the synthetic linearpolyamide fibers is within the range 0.01" to 8.0" and the length of thepore-forming fibers is within the range of 0.01" to 2.5".

4. The process-of claim 1 wherein the pore-forming fibers are celluloseacetate.'

5. The process of claim 1 wherein the pore-forming fibers are polyvinylalcohol.

References Cited in the file of this patent UNITED STATES PATENTS GreatBritain Dec. 31, 1946

